1. Field of the Invention
The present invention relates to an electron-beam dimension measuring apparatus and an electron-beam dimension measuring method for observing a sample by irradiating the sample with an electron beam.
2. Description of the Prior Art
In a process of manufacturing a semi-conductor device, a sample is observed, and a line width and the like of a pattern are measured, with use of an electron beam apparatus such as an electron microscope. When a sample is observed and measured with use of an electron beam apparatus, the electron beam apparatus scans a portion to be observed while irradiating the portion with the electron beam, and thus converts the amount of secondary electrons and the like to luminance. As an image, the converted luminance is displayed on a display unit.
Such irradiation of the sample with the electron beam in the observation and measurement of the sample causes a phenomenon that a surface of the sample is electrostatically charged. Specifically, the surface which is irradiated with the electron beam is positively or negatively charged depending on the difference in electric charge between charged particles incident onto the sample and charged particles emitted from the sample. Once the surface of the sample is charged, the secondary electrons emitted from the sample are accelerated, or are drawn back to the sample. This changes efficiency of the secondary electron emission, and causes a problem that the an image of the surface of the sample fluctuates. In addition, if the electrostatic charge continues on the surface of the sample, primary electron beams may be deflected, and accordingly the image may be distorted.
To overcome these problems, various proposals have been made on a method of preventing electrostatic charge of a sample.
As a related technique, Japanese Unexamined Patent Application Publication No. Hei 4-218941 discloses an apparatus irradiating a gas in a housing with ultraviolet light, thus ionizing the gas, and blowing the ionized gas on a sample, thereby neutralizing the sample. In addition, Japanese Patent Application Publication No. 2005-174591 discloses an apparatus irradiating a sample with ultraviolet light, thus causing the irradiated sample to emit photoelectrons, thereby neutralizing the negative charge of the sample.
When a sample is observed with use of an electron-beam apparatus, as described above, irradiation of a sample with an electron beam causes a phenomenon that the sample is electrostatically charged. However, in a case where a sample can be electrically connected, for instance, like a wafer, it is possible to ground a conductor on the electrically connected wafer to prevent the phenomenon that the sample is electrostatically charged. For this reason, no specific problem occurs in this case.
However, there are such cases that the sample may be non-conductive, and that the sample cannot be grounded even though a conductive material is used for the sample. In these cases, the sample is in an electrically floating state, causing the sample to be electrostatically charged.
For instance, when the dimension of a photomask used as a master disc for exposing a semiconductor to light is measured, the sample is electrostatically charged in the following two states: the first state in which a conductor such as chromium is present entirely on a glass substrate in the middle of a step of manufacturing interconnections, and resist interconnections for etching interconnections in the chromium are present on the conductor; and the second state in which the step of manufacturing the interconnections is completed, so that the interconnections made of the conductor such as chromium are present on the glass substrate.
In particular, immediately before the chromium starts to be etched, the layer made of the conductor such as chromium is present entirely on the glass substrate. When a portion on the conductor layer is irradiated with an electron beam, once electrostatic charge occurs, the entire conductor layer on the substrate is electrostatically charged. This electrostatic charge adversely affects observation of any other portion and measurement of a dimension of the portion. Furthermore, even when the amount of electrostatic charge of a single portion is very slight, if irradiation is performed on hundreds to thousands of portions with an electron beam, the total amount of electrostatic charge of the conductor layer eventually becomes large. This makes the dimension measured at the beginning lose its correlation with the dimension measured at the last.
An experiment by the inventors revealed that, as an example of this dimensional change, a 2-μm dimension changes by 6 nm when the electric potential of the surface of the sample changes by 10 V, where the irradiation energy of an electron beam is 1500 eV.
However, an electron-beam dimension measuring apparatus is required to have the dimensional accuracy that an error is equal to or less than 1 nm when the dimension is 2 μm. For this reason, in a case where the irradiation energy is 1500 eV, an allowable change in electric potential of the surface of the sample is approximately 1.7 V at maximum.
Furthermore, when the dimension of a resist material is measured in a step included in an interconnection rendering process, a common practice is that the irradiation energy of the electron beam is reduced to approximately 500 eV for the purpose of avoiding damage to the resist material. In this case, the change in electric potential of the surface of the sample needs to be equal to or less than approximately 0.6 V for the purpose of satisfying the requirement of the dimensional accuracy.
Moreover, depending on the resist materials, the irradiation energy needs to be reduced to approximately 400 eV or 300 eV because some resist materials have higher damage sensitivity to the electron beam. For this reason, it is desirable that the change in electric potential of the surface of the sample should be close to 0 V.
Note that in the method of controlling the electrostatic charge of the surface of the sample by use of ultraviolet light, it is difficult to keep the electric potential constant at 1 V or less on the surface of the sample with high accuracy.